Commercial processes for making starch hydrolyzates usually involve liquefying a slurry of refined starch by means of an alpha-amylase enzyme and, subsequently, subjecting the liquefied product to the action of a saccharifying enzyme such as glucoamylase. The products obtained by this method have a high content of the monosaccharide glucose. The process is of importance because the glucose-containing syrups are useful in their own right as sweeteners and constituents of many food products. The syrups are also useful as intermediates in the production of such products as high fructose syrups.
The starting materials commonly used to prepare starch hydrolyzates are refined starches such as corn starch obtained by the wet milling of the whole corn grain.
There would be a considerable economic advantage if crude starch-containing substances rather than refined starch could be used as the starting material for making starch hydrolyzates. Crude starch materials, such as broken polished rice, are readily available in certain countries as a by-product of the manufacture of polished rice.
Previous attempts to use such unrefined starting materials have met with limited success. Crude starch-containing materials contain associated proteins that are converted to soluble products during the starch hydrolysis. These soluble proteins are difficult to remove and lend undesirable color and taste properties to the final product. These impurities also interfere with the conversion of the starch hydrolyzates to high fructose syrups.
Early attempts to prepare glucose-containing syrups by hydrolysis of rice grains made use of acid hydrolysis of the starch. This acid treatment not only hydrolyzed the starch but hydrolyzed the protein as well giving materials that either could not be purified or which required very costly purification procedures.
More recently, Tegge and Richter proposed a process for the use of sorghum and broken rice as raw materials for the production of glucose (Starke, 34, 386-390 (1982). They concluded, however, that the products were of lower purity than those obtained using refined starch as the starting material. Even after the products were purified extensively, they were not suitable for conversion to fructose syrup with an immobilized glucose isomerase. A further method (Holt, et al, U.S. Pat. No. 3,910,820) used maize (corn) grits as the source of crude starch. Corn grits are obtained by dry milling corn to remove much of the fiber and oil-bearing material. The process disclosed by Holt, et al included a water tempering of the grits, a liquefaction step followed by an acidification and filtration before the saccharification step was carried out. The protein content of the product was not given, but it was necessary to refine it extensively before it could be converted to a fructose syrup. This process required two additions of alpha-amylase to accomplish the liquefaction, and removal of protein was essential before the saccharification step could be carried out because of protein solubilization during the process.
We have now discovered a process for preparing starch hydrolyzates that causes little solubilization of the protein present in crude starch materials. It can be carried out with the addition of only one portion of alpha-amylase enzyme and does not require a filtration step to remove protein before saccharification.